Plastic containers equipped with an easy-peelable lidding material are used widely as containers for a wide variety of food and drinks such as pot noodle, jelly, yogurt and fermented bean curd and pharmaceuticals. Various materials have been proposed for the sealants which can be extrusion-laminated to be used for the sealing layer of such easy-peelable lidding materials, and have been commercialized up to the present. For example, compositions comprising an ethylene/vinyl acetate copolymer or an ethylene/ethyl acrylate copolymer and a tackier resin and compositions obtained by blending polyethylene or low-crystalline ethylene/α-olefin copolymer, for example, to such compositions are known to become sealants having both sealability and easy-peelability. (For example, Japanese Patent Publication No. SHO 63-29894 and Japanese Patent Publication No. HEI 1-3895.)
However, the number of the types of plastics materials used for the container bodies has increased, and there are some new plastics materials whose sealing requirements cannot be met by the conventional sealants. As a result, there is a demand for new sealants to become available. Besides, even with containers using the existing plastics materials, the heat-sealing requirements desired of them are different depending on applications and other new properties are required of them in some cases. There is also a demand for sealants meeting these requirements.
For example, polyethylene, polypropylene, polystyrene and polyester, etc., have long been used as container materials. However, on account of such problems as transparency, heat resistance, health, strength and cost, polypropylene has come into wider use. From the viewpoint of sealability and peelability, the lidding materials for such polypropylene containers have been required to have heat sealability at low temperatures, show low temperature dependence of heat-sealing strength, have proper heat-sealing strength to the container bodies, give a good feeling of peeling at the time of unsealing and have the peeling section having excellent appearance.
Furthermore, in the application areas of packaging pudding, jelly, fruits preserved in syrup, etc. if a container is packed with contents to the brim of the mouth of the container, the contents may spill out of the container at the time of peeling. For this reason, such container is sealed and packed by packing the contents so that there is some empty space between contents and lidding material. After packing and packaging, the container is subjected to boiling treatment in hot water for several tens of minutes at 85° C. to 95° C. for the purpose of sterilization. Consequently, in these application areas, it has been required that the sealing section should not be broken due to the expansion of the air in the empty space in the container at the time of boiling treatment and that there should be no change in sealing strength due to boiling treatment.
Moreover, in the oily food packaging application area, the sealants are required to have excellent oil resistance. Especially in those application areas in which retort treatment is given to the packages, there has been a demand for easy-peelable sealants that show adequate oil resistance at temperatures above 100° C.
Furthermore, there has been a demand for those small-capacity polypropylene containers which have a relatively low heat-sealing strength, show uniform strength in peeling and give a soft feeling at the time of peeling.
However, the fact is that no such sealant for use for polypropylene containers has been found that will meet the requirements as described above. For example, the easy-peelable sealants used in the prior art mentioned above posed a problem in the application areas requiring boiling and retort treatment as mentioned above or oil resistance in some cases because they generally had a low softening point and showed inadequate oil and heat resistance. Especially, with almost all of the conventional easy-peelable sealants, the amount of evaporation residues as determined in an n-heptane extraction test, a general hygienic test, was too large, and those materials were unfit to be used under temperature conditions exceeding 100° C. in the oily fool application area in some cases.